Philadelphia, where it is

The climate of Philadelphia is continental, with cold winters and hot summers. Philadelphia is the largest city of Pennsylvania, and is located halfway between New York and Washington, and about 90 km (55 miles) from the Atlantic coast. The climate is similar to that of New York, however, being a little further away from the sea, it is a little less rainy than New York (and less snowy in winter), and a bit warmer in summer as well.
The daily average temperature ranges from 0.5 °C (33 °F) in January to 25.5 °C (78 °F) in July. Here are the average temperatures.
Philadelphia - Average temperatures
Min (°C)-4-217121821201694-1
Max (°C)571218232831302619137
Min (°F)252834455464706861483930
Max (°F)414554647382888679665545

The city is located in an area where different air masses collide: the cold ones coming from Canada and the warm ones coming from the Gulf of Mexico. The result is an unstable climate, with waves of bad weather alternating with clear and sunny days. The wind blows frequently as well.
Precipitation is quite abundant: in one year, more than 1,000 millimeters (41 inches) of rain fall. Here is the average precipitation.
Philadelphia - Average precipitation

Winter, from December to February, is cold: the average temperature is just above freezing (0 °C or 32 °F), but cold spells, during which the temperature can drop to about -10/-15 °C (5/14 °F), are not rare. The wind chill can make the air feel colder than it actually is. The temperature dropped to -20 °C (-4 °F) in January 1977, -19 °C (-2 °F) in February 1979, -21 °C (-6 °F) in January 1985, and -20.5 °C (-5 °F) in January 1994.
Snowfalls are frequent, and sometimes can be abundant, resulting in real snowstorms. On average, 57 centimeters (22.5 inches) of snow fall annually. On other occasions, the wind can blow from the south, bringing some mild days, with highs around 12/15 °C (54/59 °F). There are also clear and sunny days, with cold nights, and daytime temperatures mild or in any case acceptable.

In spring, from March to May, the weather continues to be unstable: the first warm days alternate with the return of cold weather. In March, it can snow, and this can sometimes happen still in early April. But already in April, the temperature can sometimes exceed 25 °C (77 °F). By May, the average temperature becomes pleasant, although there can still be cool days, or even a little cold and rainy, while the first heat waves can bring the temperature above 30 °C (86 °F). On sunny days, the first afternoon thunderstorms can erupt.

Summer, from June to August, is hot and humid, especially in July and August. There can be days with pleasant temperatures, around 25/27 °C (77/81 °F), maybe with some rain, but more often, there are hot and sultry days, with highs around 30 °C (86 °F). On sunny days, thunderstorms can occur in the afternoon. Sometimes, even heat waves occur, when the temperature reaches 36/38 °C (97/100 °F), and remains high at night, around 23/25 °C (73/77 °F) or above. The temperature reached 40.6 °C (105 °F) in July 2010 and 39 °C (102 °F) in July 2011.

Autumn, from September to November, is milder and less unstable than the other seasons, at least in the first part. In early September, the weather is often still characterized by heat and humidity, with sunshine and some afternoon thunderstorms, then it quickly becomes mild and relatively quiet until mid-October, while in the second half of October, the first cold days may occur. November is by now a cold month, during which the first snowfalls may occur. There is a moderate amount of rainfall also in autumn, but it is still the season in which rainy days are a little less frequent.

Benjamin Franklin Parkway

The amount of sunshine in Philadelphia is good in summer, but it's not low even in winter because, as mentioned, some sunny days can always occur. Here are the average sunshine hours per day.
Philadelphia - Sunshine

The sea at Long Beach on the Atlantic to the coast east of Philadelphia (see Atlantic City, Ocean City, Avalon) is not very warm, however, it can be considered as warm enough for swimming in July and August, when the water temperature reaches 22/23 °C (72/73 °F), while it becomes definitely cold in winter. Here are the average sea temperatures near Atlantic City.
Atlantic City - Sea temperature
Temp (°C)6446111722232117139
Temp (°F)433939435263727370635548

Best Time

The best times to visit Philadelphia are spring and autumn, in order to avoid both the winter cold and the summer heat, and in particular, the month of May and from mid-September to mid-October. June is often a good month as well, but sometimes it can be hot. In early September, the weather can be sticky, though not as hot as in midsummer. In April, it can sometimes be cold and the weather can be unstable, especially at the beginning of the month, while the second half of the month is usually mild.